1. Field of the Invention
The present invention relates to an image forming apparatus and more particularly to an image forming apparatus having a charging device.
2. Description of the Background Art
Some image forming apparatuses (for example, MFPs (Multi Function Peripherals) having a scanner function, a facsimile function, a copy function, a printer function, a data communication function, and a server function, facsimile machines, copiers, and printers) include charging devices. Such an image forming apparatus can use charge power output from a charging device, in place of electricity from a commercial power supply, in each unit of the apparatus. The charge power can be used in operation in an energy-saving mode, for example, during a sleep mode.
Document 1 below discloses an image forming apparatus using an auxiliary power supply having a secondary battery as one of power supplies for a scanner unit. When scanning a document, the image forming apparatus determines whether to supply auxiliary power or to supply AC power to the scanner unit depending on the amount of stored power in the auxiliary power supply.
Document 2 below discloses a printer including a battery and capable of print operation with power supply from the battery. The printer allows power supply from the battery depending on the remaining battery level so that the print operation is not interrupted halfway through printing.    [Document 1] Japanese Laid-Open Patent Publication No. 2007-5944    [Document 2] Japanese Laid-Open Patent Publication No. Hei 9-109518
In such an image forming apparatus having a charging device, the more are the loads to be supplied with charge power, the larger is the amount of power the charging device needs to store. To store the large amount of power, it is necessary to use, for example, a secondary battery having a large capacity. Then, the cost required for the charging device increases. In this regard, to prevent the cost increase, it is desirable to use a secondary battery having a small capacity, for example. When a secondary battery having a small capacity is used, the number of power supply targets should be minimized. In addition, when the charge remaining amount of the secondary battery reaches a predetermined value or lower, the power supply from the charging device should be stopped and switched to power supply from AC power. In this case, however, the energy-saving performance as expected with the use of the charging device is impaired. In view of such a trade-off between the low cost performance and the energy-saving performance, it is desired to improve both of them.
Neither Document 1 nor Document 2 above discloses an effective solution to this problem.